Tubes having internal diameters in the nanometer range

ABSTRACT

Hollow fibers having an internal diameter from 1 nm to 100 nm can be produced by coating degradable materials with nondegradable materials and then degrading the degradable materials. The hollow fibers are useful in separation technology, catalysis, microelectronics, medical technology, materials technology or the clothing industry.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to nanotubes, i.e., tubes or hollow fibershaving an internal diameter in the nanometer range, to a process fortheir production and to the use of these tubes or hollow fibers.

[0003] 2. Discussion of the Background

[0004] Hollow fibers, mesotubes and nanotubes are generally tubes havingan internal diameter of less than 0.1 mm.

[0005] Tubes or hollow fibers having a small internal diameter are knownand are employed in particular for separation duties, for example inmedical dialysis, for gas separation or osmosis of aqueous systems, forexample for water treatment (see Kirk Othmer, Encyclopedia of ChemicalTechnology, 4^(th) Ed., Vol. 13, pp. 312-313). The fiber materialusually comprises polymers, which may in addition have pores, i.e.properties of semipermeable membranes. The hollow fibers used forseparation duties usually have a surface area of 100 cm²/cm³ of volumecoupled with an internal diameter of from 75 μm to 1 mm.

[0006] A further application of hollow fibers is in micro electronics.Here, superconducting fibers about 60 μm in diameter are produced usingsuperconducting material by filling hollow polymeric fibers with amaterial which, after thermodegradation of the polymer, possessessuperconducting properties (J. C. W. Cien, H. Ringsdorf et al., Adv.Mater., 2 (1990) p. 305).

[0007] Tubes having a small internal diameter are generally produced byextrusion spinning processes. A number of extrusion spinning processesare described in Kirk-Othmer, Encyclopedia of Chemical Technology,4^(th) Ed., Vol 13, pp. 317-322.

[0008] Extrusion spinning processes provide hollow fibers having aninternal diameter of down to 2 μm. However, the production of hollowfibers having smaller internal diameters is not possible by theseprocesses.

[0009] Very thin fibers without internal cavity can be produced byelectrostatic spinning, or electrospinning. In electrospinning, polymermelts or polymer solutions are extruded through cannulas under a lowpressure in an electric field. The principles of this technique aredescribed, for example, in EP 0 005 035, EP 0 095 940, U.S. Pat. No.5,024,789 or WO 91/01695. Electrospinning provides solid fibers whichare 10-3000 nm in diameter; but not hollow fibers.

[0010] Hollow fibers having a very small internal diameter have hithertoonly been obtainable by electrochemical synthesis, as described in L. A.Chernozantonskii, Chem. Phys. Lett. 297, 257, (1998), by methods ofsupra molecular chemistry (S. Demoustier-Champagne et al., Europ. Polym.a. 34, 1767, (1998)) or using self organizing membranes as templates (E.Evans et al., Science; Vol. 273, 1996, pp. 933-995). Hollow carbonfibers based on fullerene chemistry (carbon nanotubes) having single- ormulti-walled structures made of a single rolled-up graphite layer (layerof six-membered carbon rings fused to one another on all sides) orconcentrically arranged graphite cylinders are described for example in“Fullerenes and Related Structures”, Ed. A. Hirsch, Springer Verlag,1999, pp. 189-234 or N. Grobert, Nachr. Chem. Tech. Lab., 47, (1999).

[0011] However, these methods can only be applied to specific materialsand cannot be employed to produce industrially useful, i.e. mechanicallyand chemically stable, hollow fibers.

[0012] Hollow fibers having internal diameters in the μm range areknown. For instance, WO 97/26225, EP 0 195 353 and U.S. 5,099,906disclose hollow fibers which are composed of ceramic materials and havean internal diameter of at least 1 μm. Hollow fibers which are composedof metals and have an internal diameter of 1-1 000 μm are described inFR 12 11 581 and DE 28 23 521.

[0013] WO 01/09414 discloses meso- and nanotubes having internaldiameters in the range from 10 nm to 50 pm that are preferably producedby electrospinning. However, the electrospinning process disclosedtherein does not allow the production of smaller fibers, since a fiberproduced when the material to be spun is thinned out to any degree isirregular and has thick portions.

[0014] There are many applications, for example the separation of gases,in which it is desirable to employ hollow fibers having very smallexternal and/or internal diameters that are composed of variousmaterials optimized to the particular area of application. Moreparticularly, the materials should be capable of withstanding thermal,mechanical and chemical stresses, if desired have a porous structure,selectively be electrical conductors or insulators and be composed ofpolymers, inorganics or metals.

SUMMARY OF THE INVENTION

[0015] It is therefore an object of the present invention to providehollow fibers of industrially usable materials that have an internaldiameter in the nm range.

[0016] This and other objects have been achieved by the presentinvention the first embodiment which includes a hollow fiber having aninternal diameter from 1 nm to 100 nm and an outer wall comprising ametal-containing inorganic compound, a polymer, a metal or a combinationthereof.

[0017] In another embodiment the present invention provides a processfor preparing a hollow fiber, comprising:

[0018] coating a fiber of a first, degradable material with at least onecoating of at least one second material; and

[0019] degrading the first, degradable material to obtain the hollowfiber;

[0020] wherein said hollow fiber has an internal diameter from 1 nm to100 nm.

[0021] In yet another embodiment the present invention relates to amethod for removal of a metabolite or an enzyme from cytoplasm,comprising:

[0022] contacting said cytoplasm with the hollow fiber.

BRIEF DESCRIPTION OF DRAWINGS

[0023]FIG. 1 shows an electrospinning apparatus, embodiments of hollowfiber, and a process for production of the hollow fiber.

[0024]FIG. 2 shows a scanning electron photomicrograph of polylactidetemplate fibers.

[0025]FIG. 3 shows a tunneling electron micrograph of polylactidetemplate fibers.

[0026]FIG. 4 shows a tunneling electron photomicrograph of polyamidetemplate fibers.

[0027]FIG. 5 shows a scanning electron micrograph of poly(p-xylene)fibers.

[0028]FIG. 6 shows a scanning electron micrograph of poly(p-xylene)fibers.

[0029]FIG. 7 shows a tunneling electron photomicrograph ofpoly(p-xylene) fibers.

[0030]FIG. 8 shows a tunneling electron micrograph of poly(p-xylene)fibers.

[0031]FIG. 9 shows a wide angle X-ray graph.

DETAILED DESCRIPTION OF THE INVENTION

[0032] It has been surprisingly found that hollow fibers having aninternal diameter in the desired run range are producible in a precisemanner from a wide variety of materials such as polymers, inorganics oreven metals.

[0033] The present invention accordingly provides a hollow fiber havingan internal diameter from 1 nm to 100 nm and an outer wall constructedof metal-containing inorganic compounds, metals and/or polymers orcombinations thereof.

[0034] The internal diameter of the hollow fibers according to theinvention is preferably in the range from 1 nm to 5 nm, more preferablyin the range from 1 nm to 9 nm and most preferably in the range from 1nm to 5 nm. The internal diameter includes all values and subvaluestherebetween, especially including 1.5, 2.5, 3, 3.5, 4 and 4.5 nm.

[0035] The hollow fiber length is determined by the intended use and isgenerally in the range of from 50 μm up to several mm or cm. The hollowfiber length includes all values and subvalues therebetween, especiallyincluding 100, 200, 300, 400, 500, 600, 700, 800, 900 μm; 1, 2, 3, 4, 5,6, 7, 8, 9 mm; 1, 2, 3, 4, 5, 6, 7, 8 and 9 cm.

[0036] The wall thickness, i.e., the thickness of the outer wall of thehollow fiber, is variable and is generally in the range from 1 to 500nm, preferably in the range of from 1 to 100 nm and more preferably inthe range from 10 to 25 nm. The thickness of the outer wall includes allvalues and subvalues therebetween, especially including 10, 50, 100,150, 200, 250, 300, 350, 400 and 450 nm.

[0037] Hollow fibers according to the present invention as well as thevery small internal diameters, have a number of properties which makethem suitable for use in the fields of medicine, electronics, catalysis,chemical analysis, gas separation, osmosis or optics.

[0038] Thus, the outer wall of the hollow fiber according to the presentinvention can be constructed from the most diverse materials, forexample from polymers, metals or metal containing inorganic compounds.The outer wall can have one layer of these materials, i.e., consistentirely thereof or have a plurality of layers composed of the same ordifferent materials. The very small internal diameter ensures a veryhigh ratio of hollow fiber surface area to volume which can be between500 and 2 000 000 cm²/cm³, preferably in the range from 5 000 to 1 000000 cm²/cm³ and more preferably in the range from 5 000 to 500 000cm²/cm³. The ratio of hollow fiber surface area to volume includes allvalues and subvalues therebetween, especially including 1,000; 10,000,50,000, 100,000, 200,000, 300,000, 400,000, 500,000, 600,000, 700,000,800,000, 900,000, 1,000,000, 1,200,000, 1,400,000, 1,600,000 and1,800,000 cm²/cm³.

[0039] The metal-containing inorganic compounds of the hollow fiberaccording to the invention are for example metal oxides, metal mixedoxides, spinel, metal nitrides, metal sulfides, metal carbides, metalaluminates or metal titanates. Boron compounds or metal-doped carbonnanotubes having single- and multi-wall structures made from a singlerolled-up graphite layer (layer of six-membered carbon rings fused toone another on all sides) or concentrically arranged graphite cylindersare not metal-containing compounds for the purposes of the presentinvention. Materials similar to carbon nanotubes having concentricallyarranged polyhedral or cylindrical layer structures such as for exampleWS₂, MoS₂ and VS₂ are likewise not metal-containing compounds for thepurposes of the present invention.

[0040] For the purposes of the present invention, the polymers arepolycondensates, polyaddition compounds or products of chain growthpolymerization reactions, but not graphite-like compounds composed ofpure or doped carbon.

[0041] The present invention further provides a process for producingthe hollow fiber.

[0042] The process for producing the hollow fiber according to thepresent invention comprises coating a fiber of a first, degradablematerial with at least one further material. Subsequently, the firstmaterial is degraded in such a way that the hollow fiber thus obtainedhas an internal diameter from 1 nm to 100 nm.

[0043] In a preferred embodiment of the process, the first, degradablematerial may be admixed with 0.1-10% by weight of a basic compound, suchas pyridine which is preferable when polyamides are used and can be usedfor example as a solvent additive in electrospinning. The amount ofbasic compound includes all values and subvalues therebetween,especially including 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6,6.5, 7, 7.5, 8, 8.5, 9 and 9.5% by weight.

[0044] In another preferred embodiment of the process, the degradablematerial is admixed with 10-60% by weight and preferably 25-50% byweight of a noble metal salt. The amount of noble metal includes allvalues and subvalues therebetween, especially including 20, 30, 40 and50% by weight. Preference is given to platinum, nickel, cobalt, rhodiumand palladium salts of organic acids, such as acetate or formate. It isalso possible to add the hereinbelow specified metals of groups I toXIII, a and b of the periodic table. This embodiment of the processmakes is possible to produce nanotubes having small noble metal crystalson the inner surface. These nanotubes are especially useful ascatalysts. It is also possible to apply the above two embodiments of theprocess at the same time.

[0045] Preferred embodiments of the hollow fiber and of the process forproduction thereof are illustrated in FIG. 1 (b and c).

[0046] In another embodiment preferred of the process, the initial stepcomprises subjecting a fiber (FIG. 1, b, I) composed of a first,degradable material to a coating operation (FIG. 1, b, II). This fibermay be composed of a material which is degradable thermally, chemically,radiochemically, physically, biologically or by means of plasma,ultra-sound or extraction with a solvent. These fibers may be producedby electrospinning.

[0047] Details concerning electrospinning technology are described, forexample in D. H. Reneker, I. Chun., Nanotechn. 7, 216 (1996). The basicconstruction of an electrospinning apparatus is shown in FIG. 1 (a).

[0048] The diameter of the degradable fibers should be in the same orderof magnitude as the later desired internal diameter of the hollowfibers. In general, the later cavity in the hollow fibers is ofapproximately the same size as the diameter of the degradable fibers orcoatings. The precise dimensions depend on the materials used and theirchanges during the degradation operation and can be determined withoutdifficulty by preliminary experiments. Useful degradable fiber materialsinclude organic or inorganic materials, especially polymers such aspolyesters, polyethers, polyolefins, polycarbonates, polyitrethanes,natural polymers, polylactides, polyglycosides, poly-a-methylstyreneand/or polyacrylonitriles. The electrospinning process also makes itpossible to produce multicomponent fibers, i.e., fibers having differentmaterials in different layers or fibers having a certain surfacetopography, i.e., having smooth or porous surfaces.

[0049] The surface finish of the fiber or layer of degradable materialsalso determines the surface topography of the subsequent coatings. If,for example, a rough or micro structured inner surface is desired forthe hollow fibers, this can be achieved by means of a correspondinglyrough fiber of a degradable material. Rough or microstructured fiberscan be obtained by electrospinning a polymer solution containing avolatile solvent. Furthermore, additives such as salts, for examplesodium sulfate, metallic nanopowders, conductive polymers such aspolypyrroles or graphite can distinctly enhance the conductivity of thespun material.

[0050] The coating with the at least one further nondegradable materialcan be effected by gas phase deposition, plasma polymerization orapplication of the material in a melt or in solution. The coating can beeffected in various layers and using various materials and forms theouter wall of the hollow fiber.

[0051] This coating, i.e. the construction of the outer walls can beeffected for example by gas phase deposition, knife coating, spincoating, dip coating, spraying or plasma deposition of polymers such aspoly(p-xylylene), polyacrylamide, polyimides, polyesters, polyolefins,polycarbonates, polyamides, polyethers, polyphenylene, polysilanes,polysiloxanes, polybenzimidazoles, polybenzothiazoles, polyoxazoles,polysulfides, polyester amides, polyarylenevinylenes, polylactides,polyether ketones, polyurethanes, polysulfones, ormocers, polyacrylates,silicones, aromatic copolyesters, poly-N-vinylpyrrolidone,polyhydroxyethyl methacrylate, polymethyl methacrylate, polyethyleneterephthalate, polybutylene terephthalate, polymethacrylonitrile,polyacrylonitrile, polyvinyl acetate, neoprene, Buna N, polybutadiene,polytetrafluoroethene, cellulose (modified or unmodified), alginates orcollagen, homopolymers or copolymers and/or blends thereof.

[0052] Furthermore, the degradable layers or fibers may be coated with afurther material obtained by polymerization of one or more monomers.Preferred monomers for the homo- or copolymerization include, forexample, methacrylate, styrene, styrenesulfonate, 1,6-hexamethylenediisocyanate (HDI), 4,4′-methylenebiscyclo-hexyl-diisocyanate (HMDI),4,4′methylenebis-(benzyl-diisocyanate) (MDI), 1,4-butanediol,ethylenediamine, ethylene, styrene, butadiene, 1-butene, 2-butene, vinylalcohol acrylonitrile, methyl methacrylate, vinyl chloride, fluorinatedethylenes and/or terephthalates.

[0053] The coating, i.e., the construction of the outer wall of thehollow fibers, can be composed of metals of the groups la, Ib, Ia, IIb,IIa, IIIb, IVa, IVb, Vb, VIb, VIIb and/or VIIIb of the periodic table,each as a pure metal or as an alloy. Preferred metals include forexample gold, palladium, aluminum, platinum, silver, titanium, cobalt,ruthenium, rhodium, sodium, potassium, calcium, lithium, vanadium,nickel, tungsten, chromium, manganese and/or silicon. The coating can beeffected by vapor deposition of the metals or by decomposition ofsuitable organometal-containing compounds using chemical vapordeposition (CVD) methods.

[0054] Polymeric coating materials may further bear functional groupssuch as esters, amides, amines, silyl groups, siloxane groups, thiols,hydroxyl groups, urethane groups, carbamate groups, nitrite groups, C═Cgroups, C═C groups, carboxylic acid halide groups, sulfoxide groups,sulfone groups, pyridyl groups, arylphosphine groups or else ionicgroups such as carboxylic acids, sulfonic acids or quaternary amines.The functional groups can be attached to the inner and/or outer surfaceof the hollow fibers to improve the surface properties of the hollowfibers in separation or osmosis processes. The functional groups canalso be chemically modified subsequently by polymer-analogous reactions,for example by hydrolysis of esters.

[0055] Appropriate functionalization, furthermore, can be used to haveactive ingredients such as antibiotics, anesthetics, proteins such asinsulin, antifouling agents and agrochemicals such as herbicides orfungicides reversibly fixed in the hollow fibers and/or graduallyreleased again in specific concentrations at a controlled rate.

[0056] The outer wall of the hollow fibers, i.e., the nondegradablefurther material, can also be constructed of glass, glass ceramics,SiO_(x), perovskite, ceramics, aluminas or zirconias, optionally ofsilicon carbide, boron nitride, carbon and metal oxides. Useful methodshere likewise include gas phase deposition processes (CVD or physicalvapor deposition (PVD)) or else hydrothermal processes.

[0057] Preferred perovskites have the general formula

LaXYMgO

[0058] wherein X=Ca, Sr or Ba; and Y=Ga or Al; (without stoichiometry)which have oxygen ion conductance properties.

[0059] The degradation of the degradable material can be effectedthermally, chemically, radiation-induced, biologically, photochemicallyor by means of plasma, ultrasound or extraction with a solvent. Thermaldegradation is particularly preferred in practice. The decompositionconditions vary with the material ranging from 100 to 500° C. and from0.001 mbar to 1 bar, particularly preferably from 0.001 to 0.1 mbar. Thedecomposition temperature includes all values and subvaluestherebetween, especially including 150, 200, 250, 300, 350, 400 and 450°C. The decomposition pressure includes all values and subvaluestherebetween, especially including 0, 005, 0.01, and 0.05 mbar.Degradation of the material provides a hollow fiber whose wall materialis composed of the coating materials.

[0060] As shown in FIG. 1 (b and c), it is also possible for a pluralityof layers of different materials to be applied to the fiber. Thisprovides hollow fibers having different inner and outer surfaces orhollow fibers where the outer walls can be constructed of a plurality oflayers. The different layers can perform different functions in that,for example, the inner layer can have particular separation propertiesfor chromatographic purposes, for example, and the outer layer can havehigh mechanical stability.

[0061] The following layer sequences for the hollow fibers according tothe invention may be mentioned by way of example:

[0062] glass/metal

[0063] metal/glass

[0064] glass/polymer

[0065] polymer/glass

[0066] polymer/polymer

[0067] metal/metal

[0068] metal-containing inorganic compound/metal-containing inorganiccompound

[0069] ceramic/ceramic

[0070] polymer/metal

[0071] metal/polymer

[0072] ceramic/polymer

[0073] polymer/ceramic

[0074] metal/ceramic

[0075] ceramic/metal

[0076] polymer/metal/polymer

[0077] metal/polymer/metal

[0078] metal/ceramic/metal

[0079] polymer/ceramic/polymer

[0080] ceramic/polymer/ceramic

[0081] polymer/glass/polymer

[0082] glass/polymer/glass.

[0083] Hollow fibers according to the present invention are useful inparticular as a separation or storage medium for gases, liquids orparticle suspensions and for filtering or purifying compositions ofmatter. Preferred applications here are as a membrane for gases,especially H₂ or liquids, for particle filtration, in chromatography,for oil/water separation, as an ion exchanger in dialysis, for sizeseparation of cells, bacteria or viruses, as a constituent of anartificial lung, for desalination, for drainage or irrigation or as afilter for dewatering power fuels.

[0084] Hollow fibers according to the invention may further be used insensor technology for solvent, gas, moisture or biosensors, in capillaryelectrophoresis, in catalytic systems, in scanning probe microscopy oras materials of construction in superlightweight building construction,as a mechanical reinforcement similar to glass fibers, as a noise orvibration abate, as a composite material or filler, as a controlledrelease or drug delivery system, in medical separation technologies, indialysis, as an artificial lung, as a protein store or in tissueengineering.

[0085] The hollow fibers according to the invention may be used in theclothing/textile industry as a thennal insulator in clothing or sleepingbags, in photo- or thermochromic clothing through embedding of dyes inthe tube interior or as an authenticator through markers in the tubesinterior.

[0086] Hollow fibers according to the invention also find use inelectronics, optics or energy production the hollow fibers can be usedto produce wires, cables or capacitors, micromachines (for example forpiezoelectric, shaping, nanoperistaltic pumps or for shapingphotoadrressable polymers) or interlayer dielectrics. Further uses forhollow fibers according to the invention are microreactors, for examplefor catalytic reactions, template reactions and bioreactions, heatgeneration through conversion of sunlight (solar a systems) or in chiptechnology as flexible devices or microscopy as a sensor constituent(for example; as tips or probes for scanning probe microscopes br SNOMinstruments).

[0087] The hollow fibers according to the present invention have a verylow dielectric constant and therefore can also be used as a dielectric,in particular as an interlayer dielectric in electronic components. Forexample, in chip manufacture, interlayer dielectrics having a lowdielectric constant are important in the production of new chipgenerations having even smaller dimensions or higher storage densitiesdue to the high proportion of included air per unit volume. The hollowfibers according to the invention have a dielectric constant of lessthan 4, preferably less than 3, most preferably less then 2, ideallyless than 1.5.

[0088] The hollow fibers are preferably used as a web or mat fordielectric applications due to the large surface area of the hollowfibers according to the invention, these can also be used in fuel cells,batteries or electrochemical reactions. For such uses, the outer wall ofthe hollow fibers is advantageously composed of oxygen ion conductors,for example perovskites. In oxidation reactions, the hollow fibers maybe surrounded by the reactant, an olefin for example, while oxygen ispassed through the cavities of the fibers. The oxidation product isformed on the outside of the hollow fibers and transported away.

[0089] The hollow fibers according to the present invention can be usedas a catalytic system. It is thus possible, for example, to use hollowfibers composed of noble metals such as platinum or palladium asdenoxing catalysts in motor vehicles.

[0090] Hollow fibers according to the invention which are composed ofcell-compatible materials or have appropriately modified surfaces can beincorporated introduced into cell membranes and used for the separationand also recovery or removal of metabolites, enzymes and othercomponents of the cytoplasm within cells or cytoplasmic components andhence for the recovery of biopharmaceuticals.

[0091] Having generally described this invention, a furtherunderstanding can be obtained by reference to certain specific exampleswhich are provided herein for purposes of illustration only, and are notintended to be limiting unless otherwise specified.

EXAMPLES Example 1 Production of Polylactide Template Fibers byElectrospinning

[0092] A 5% solution of poly-L-lactide in dichloromethane (conductivity<10⁻⁷ μs/cm) containing 50% by weight of Pd(OAc)₂, based on thepolylactide, was electrospun at a voltage of 48 kV in the apparatus ofFIG. 1 (a). The separation of the cannula tip (diameter 0.3 mm) from thesubstrate plate (glass) was 10 cm. The fibers were further used withoutfurther treatment. A scanning electron photomicrograph of the fibers isshown in FIG. 2. FIG. 3 shows a tunneling electron micrograph of thematerial thus obtained.

Example 2 Production of Polyamide Template Fibers by Electrospinning

[0093] An 8% solution of nylon 46 containing 2% by weight of pyridine,based on N46, in formic acid was electrospun at a voltage of 55 kV inthe apparatus shown in FIG. 1(a). The separation of the cannula tip(diameter 0.3 mm) from the substrate plate (glass) was 15 cm. The fiberswere further used without further treatment. A tunneling electronphotomicrograph of the fibers is shown in FIG. 4.

Example 3 Production of poly(p-xylylene) Hollow Fibers by Coating fromthe Gas Phase

[0094] Polyamide template fibers produced by electrospinning as perExample 1 were placed in a gas phase deposition apparatus. Subsequently37 mg of analytically pure [2.2] paracyclophane were evaporated at 220°C./0.1 mbar and pyrolyzed at 800° C., causing the formation of 25poly(p-xylene) (PPX) in the sample space at about 20° C.

[0095] The poly(p-xylylene) polylactide composite fabric was extractedwith chloroform for 24 hours. The formation of poly(p-xylylene) hollowfibers having an internal diameter from about 6 to 20 nm was confinmedby scanning electron microscopy (FIGS. 5, 6).

Example 4 Production of poly(p-xylylene) Hollow Fibers by Coating fromthe Gas Phase

[0096] Polyamide template fibers produced by electros: pinning as perExample 2 were placed in a gas phase deposition apparatus. Subsequently37 mg of analytically pure [2.2]paracyclophane were evaporated at 220°C./0.1 mbar and pyrolyzed at 800° C., causing the formation ofpoly(p-xylylene) (PPX) in the sample space art about 20° C.

[0097] The poly(p-xylylene) polyamide composite was extracted withfonmic acid for 24 hours. The formation of the hollow fibers having aninternal diameter of 45 nm is discernible from the tunneling electronphotomicrograph in FIG. 7.

Example 5 Production of poly (p-xylylene)/Hollow Fibers by Coating fromthe Gas Phase

[0098] Polylactide template fibers produced by electrospinning as perExample 1 were placed in a gas phase deposition apparatus. Subsequently40 mg of analytically pure [2.2]paracyclophane were evaporated at 220°C./10.1 mbar and pyrolyzed at 700° C., causing the formation ofpoly(p-xylylene) in the sample space at about 20° C. Thepoly(p-xylylene) polylactide composite fabric was thenmally treated in avacuum oven at 285° C./0.01 mbar for 8 hours. The fonmation ofpoly(p-xylylene)/hollow fibers laving an average internal diameter ofabout 17 nm was confirmed by scanning electron microscopy (FIG. 8).

[0099] Thermal degradation gives palladium crystallites 4-10 nm in sizeon the inner surface of the tube is shown in FIG. 8. FTIR spectroscopyconfirms the degradation of the polylactide. The conversion of palladiumacetate to metal-containing palladium is confirmed by wide angle X-rayspectroscopy (FIG. 9).

[0100] German patent application 10133393.5, filed Jul. 13, 2001, isincorporated herein by reference.

[0101] Numerous modifications and variations on the present inventionare possible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

1. A hollow fiber having an internal diameter from 1 nm to 100 nm and anouter wall comprising a metal-containing inorganic compound, a polymer,a metal or a combination thereof.
 2. The hollow fiber according to claim1, wherein said internal diameter is from 1 nm to 10 nm.
 3. The hollowfiber according to claim 1, wherein said outer wall comprises ahomopolymer, a copolymer or a blend of compounds selected from the groupconsisting of poly(p-xylylene), polyacrylamide, polyimides, polyesters,polyolefins, polycarbonates, polyamides, polyethers, polyphenylene,polysilanes, polysiloxanes, polybenzimidazoles, polybenzothiazoles,polyoxazoles, polysulfides, polyester, amides, polyarylenevinylenes,polylactides, polyether ketones, polyurethanes, polysulfones, ormocers,polyacrylates, silicones, aromatic copolyesters,poly-N-vinylpyrrolidone, polyhydroxyethyl methacrylate, polymethylmethacrylate, polyethylene terephthalate, polybutylene terephthalate,polymethacrylonitrile, polyacrylonitrile, polyvinyl acetate, neoprene,Buna N, polybutadiene, polytetrafluoroethene, modified cellulose,unmodified cellulose, alginates, and collagen.
 4. The hollow fiberaccording to claim 1, wherein said outer wall comprises a metal or analloy of metals selected from the group consisting of metals of groupsIa, Ib, Ia, IIb, IIIa, IIIb, IVa, IVb, Vb, VIb, VIb, VIIb of theperiodic table, and mixtures thereof.
 5. The hollow fiber according toclaim 1, wherein said outer wall comprises glass, glass ceramics,SIO_(x), perovskite, ceramics, aluminas or zirconias.
 6. The hollowfiber according to claim 1, wherein said outer wall comprises aplurality of layers.
 7. The hollow fiber according to claim 1, having adielectric constant of less than
 4. 8. A process for preparing a hollowfiber, comprising: coating a fiber of a first, degradable material withat least one coating of at least one second material; and degrading thefirst, degradable material to obtain the hollow fiber; wherein saidhollow fiber has an internal diameter from 1 nm to 100 nm.
 9. Theprocess according to claim 8, wherein the first, degradable materialcomprises 10-60% by weight of a noble metal salt.
 10. The processaccording to claim 8, wherein the first, degradable material furthercomprises a basic compound.
 11. The process according to claim 8,wherein the second material comprises an inorganic compound, a polymer,a metal or a mixture thereof.
 12. The process according to claim 8,wherein the second material comprises homopolymers, copolymer or blendsof compounds selected from the group consisting of poly(p-xylylene),polyacrylamide, polyimides, polyesters, polyolefins, polycarbonates,polyamides, polyethers, polyphenylene, polysilanes, polysiloxanes,polybenzimidazoles, polybenzothazoles, polyoxazoles, polysulfides,polyester amides, polyarylenevinylenes, polylactides, polyether ketones,polyurethanes, polysulfones, ormocers, polyacrylates, silicones,aromatic copolyesters, poly-N-vinylpyrrolidone, polyhydroxyethylmethacrylate, polymethyl methacrylate, polyethylene terephthalate,polybutylene terephthalate, polymethacrylonitrile, polyacrylonitrile,polyvinyl acetate, neoprene, Buna N, polybutadiene,polytetrafluoroethene, modified cellulose, unmodified cellulose,alginates, and collagen.
 13. The process according to claim 8, whereinthe second material comprises a metal or an alloy of metals selectedfrom the group consisting of metals of groups Ia, Ib, Ia, IIb, IIa,IIIb, IVa, IVb, Vb, VIb, VIb, VIIb of the periodic table, and mixturesthereof.
 14. The process according to claim 8, wherein the secondmaterial comprises metal oxides, glass, glass ceramics SiO_(x),perovskite, ceramics, aluminas, silicon carbide, boron nitride, carbonor zirconias.
 15. The process according to claim 8, wherein the secondmaterial is obtained by polymerization of one or more monomers.
 16. Theprocess according to claim 15, wherein the second material is obtainedby homopolymerization or copolymerization of a compound selected fromthe group consisting of methacrylate, styrene, styrene sulfonate,1,6-hexamethylene diusocyanate, 4,4′-methylenebiscyclohexyldilsocyanate, 4,4′-methylenebis(benzyl-diisocyanate), 1,4butanediol,ethylenediamine, ethylene, styrene, butadiene, 1-butene, 2-butene, vinylalcohol, acrylonitrile, methyl methacrylate, vinyl chloride, fluorinatedethylenes, terephthalate or mixtures thereof.
 17. The process accordingto claim 8, wherein the degrading of the first, degradable material iseffected thermally, chemically, biologically, radiation-induced,photochemically, by plasma, by ultrasound or by extraction with asolvent.
 18. A separation medium or storage medium for gases, liquids orparticle suspensions, comprising: the hollow fiber according to claim 1.19. A method for removal of a metabolite or an enzyme from cytoplasm,comprising: contacting said cytoplasm with the hollow fiber according toclaim 1.